The present application pertains to the art of magnetic resonance spectroscopy. More particularly, the present invention relates to the operation of magnetic resonance and other radio frequency probes above self resonance. Although the present invention finds application in the field of medical magnetic resonance imaging, it is to be appreciated that the invention may also find utility in conjunction with other types of magnetic resonance spectroscopy, other nuclear and electron resonance techniques, and other high radio frequency transmission applications.
Heretofore, magnetic resonance imaging apparatus have included a radio frequency resonator coil for inducing magnetic resonance in a region of the body of interest. When performing proton magnetic resonance imaging in the range of 1.0-2.0 Tesla and above, a corresponding operating resonance frequency of 40-90 and above megahertz was required.
The self resonance frequency of Helmholtz pair coils and saddle coils varies as a function of several factors, including diameter. When Helmholtz pair and saddle coils are configured in a sufficiently large diameter to receive a human torso therein, their self resonance point or parallel resonant frequency was well below 40 megahertz. One approach for achieving higher resonance frequencies was to use other resonator designs, particularly resonator designs which incorporated relatively large expanses of thin metal foil. See for example U.S. Pat. No. 3,783,419 issued Jan. 1, 1974 to Lafond, et al. Lumped and distributed capacitance techniques were also utilized to raise the self resonance frequency of coils and probes.
These prior art higher frequency coil or probe arrangements had several disadvantages. First, the current path through metal foil resonators was difficult to control. Eddy currents and other current irregularities destroyed RF homogeneity of the resonator assembly. Further, the inherent eddy current paths prevented the use of crossed coil or quadrature probes, because orthogonal placement was insufficient to reduce or eliminate the induced eddy currents. Further, the resonators were not amenable to tuning for operation over a wide frequency range, rather, the low inductance inherent in most resonator designs adversely affected their utility at low frequencies.
The lumped or distributed capacitance designs also had several disadvantages. First, the placement of capacitors about the coil structure prevented the coil from being resonated by conventional means below the series resonant point of the capacitors and linked coil inductance. That is, when the frequency was shifted sufficiently that the magnitude of the capacitive reactance exceeded that of the inductive reactance, the coil structure as a whole became capacitively reactive. Moreover, in larger structures, the series resonant point began to approach the self parallel resonant point. This required holding the operating range of the coil between the limits of the series and parallel resonance. Moreover, the peformance of the coil tended to become erratic. For example, small changes in parameters caused by sample loading variations were magnified.
The present invention contemplates a new and improved magnetic resonance probe assembly which permits radio frequency pickup coil systems to operate at frequencies above their natural self resonant frequencies.